The disorders of hyperthermia (also known as heat-related injury or illness) exist on a continuum of injury, which is marked by dysregulation of the body’s thermoregulatory capacity. This condition can vary both in presentation and in severity from benign conditions, including heat cramps and heat edema to life-threatening hyperthermia, which is also known as heatstroke. This article will discuss the non-life-threatening heat-related illnesses. It is essential to be able to identify and manage these conditions appropriately as moderate hyperthermia can progress to life-threatening heatstroke. As such, including these injuries in medical decision making, prompt identification, and appropriate treatment is of significant importance. This article will also review the epidemiology, including at-risk populations, red-flag features of patient presentations, treatment options and strategies, and preventative techniques, which all play a significant role in decreasing the morbidity, mortality, and healthcare costs associated with these injuries.
Humans are homeotherms, which means we are organisms capable of maintaining our body's temperature in a specific range. While baseline body temperature is between 36.5 to 37.5 C (97.7 to 99.5 F), the body has mechanisms to address temperatures between approximately 35 to 41 C (95.0 to 105.8 F), after which it can no longer self-regulate and compensate for external thermal loads. The body’s core temperature is the sum of metabolic heat production, mechanical work, and heat exchange with the environment. A balance between internal and external variables (e.g., levels of metabolically active hormones, external temperature and humidity, and the amount of environmental convection) affects the body to cause either a net gain or a net loss of temperature. Heat exchange with the environment occurs through evaporative, convective, conductive, and radiative loss. As external temperature and humidity rise, radiation and conduction become ineffective, and evaporative dissipation of heat is the primary means by which the body can cool itself.
How this process is monitored and effected is through sensory receptors. Thermoreceptors found in the skin, organs, and spinal cord relay information via afferent nerve fibers to the preoptic area of the hypothalamus. This response causes autonomic and behavioral changes that help correct body temperature back to baseline. Physiologic changes include vasodilation of blood vessels with the majority of flow occurring primarily in the peripheral arteriovenous anastomoses (AVA). The highest concentration of AVAs are present in the glabrous tissues, including the hands, feet, and face, and, as such, are important areas to address during treatment with rapid cooling. Other physiologic changes include increased rate of sweat production via sympathetic cholinergic receptors and decreased metabolism leading to decreased internal heat production. It also elicits behavioral modifications, but these are under voluntary control. Examples of these include the desire to seek shade or shelter, use air-conditioning or forced convection with fans, and loosen clothing or shed insulating layers. As these are under voluntary control, individuals are at an increased risk for heat illness when they ignore these impulses either consciously or subconsciously. An example of this is athletes who push themselves to train in hot conditions or in elderly residents who may not possess access to air-conditioning units during heat waves or the means to evacuate from an unsafe environment. Comorbidities, mental status changes, and medications may also play a role in limiting a person's response to exposure to a hot environment.
Heat illness is a common issue experienced worldwide. The National Oceanic and Atmospheric Administration have found it to be the number one cause of weather-related fatalities year after year. This data also has support from data published by the Centers for Disease Control. In the United States between 2001 and 2010, a network of 20 states had had a total of 28133 hospitalizations for heat illness, with a majority of cases occurring in males and persons 65 years or older. Of the participating states, the majority of hospitalizations occurred in the Midwest and Southern states, which have consistent exposure to higher index temperatures throughout the year. Support for this concept exists from correlations found in that data set between the number of hospitalizations and the monthly maximum temperature/heat index. These findings were not just limited to the United States but also found in many other countries all over the world. There are also gender differences with these conditions occurring more often in males. This data may be due to a disproportional number of males that engage in manual labor, including work outdoors in construction or in mine shafts, which combine physical exertion in a hot environment with limited airflow.
Risk factors for the development of heat-related illness include:
Other than the elderly who have many of the risk factors presented above, specific population groups that are susceptible to heat-related injuries include adolescents and athletes. Although heat injury appears to be more significant in the elderly due to decreased health reserves, it also frequently occurs in adolescents and teenagers during the summer months, which may be due to increased participation in activities outside, including in sports. Athletes, in general, are at considerable risk of heat illness and heat stroke, which research shows the latter of these has to be the third-highest cause of mortality in this population. The risk for all groups of people increases in these environments due in large part to a large volume of sweat loss accompanied by clinically significant sodium and chloride loss leading to dehydration.
Modifiable risk factors for the development of heat illness include obese body habitus, alcohol use, medication use, illicit stimulant drug use, hydration status, and avoidance of exertion in hot and humid environments.
Obtaining an accurate history from the patient, family, bystanders, or first responders is of pivotal importance as this can help determine what the patient was doing when symptoms started and in what environment they were found. Symptoms of heat illness vary depending on which form of heat injury has occurred. The following are a list of various heat-related injuries:
Central nervous system involvement with symptoms or signs that may include seizures, gait abnormalities, delirium, coma, etc. in the setting of elevated temperature cause concern for heatstroke, as described elsewhere.
History and physical examination may be all that is necessary to make the diagnosis, and it is not uncommon that testing, including lab work, is normal. If a patient's presentation is concerning for heat stress, heat injury, or heatstroke, a core temperature should be obtained with rectal thermometry if clinically possible. Laboratory tests are not typically necessary unless there is a concern for electrolyte imbalance, significant dehydration, or organ injury or to investigate other diagnoses. Because of the risk for critical cardiovascular collapse found with progression to heatstroke, further testing may be warranted depending on the severity of clinical presentation. Episodes that are not consistent with the above presentations should obtain a referral for further workup. Tests to consider include:
Patients may also have elevated levels of lactic acid secondary to decreased organ perfusion. Other tests, including a lumbar puncture or CT scan of the head, might be indicated if concern exists for other causes of a patient's presentation.
In general, the initial treatment of heat illness involves stopping the activity and moving the individual to a cool environment as all of these conditions will benefit from cooling. Supportive care is the recommended strategy for the more benign conditions described above, including heat edema, syncope, rash, and cramps. For more serious presentations, including heat exhaustion, resuscitation should occur as typical to address concerns that may exist for airway, respiratory, or cardiovascular compromise and, if present, may warrant emergent transportation to a higher level of care. Otherwise, oral rehydration with water or an electrolyte-containing drink can commence on-site. Studies have previously shown that oral and IV rehydration are equivalent for the treatment of heat illness. Care should be taken not to overhydrate patients as some may be sensitive to significant volume status changes, including patients with a decreased cardiac function. Specific strategies are as follows:
There is no role for antipyretics or muscle relaxants like dantrolene in heat illness.
More dramatic measures are necessary if the patient has symptoms and signs consistent with heatstroke; the management of heatstroke is beyond the scope of this article.
The differential diagnosis for these presentations includes:
Specific heat illnesses can also have symptoms that mimic other syndromes. For example, heat edema should not be mistaken for swelling caused by other medical conditions, including hypervolemia from congestive heart failure or liver dysfunction or a deep vein thrombosis of the extremity. Heat syncope should not be confused for syncope of other etiology, including cardiogenic or with different pathologies that lead to a transient change in consciousness (e.g., pulmonary embolism, rupture of an ectopic pregnancy, aortic dissection, or some environmental gas exposures).
The prognosis for the more benign heat illnesses described above is excellent. They are self-limited and resolve with removal from the hot environment and supportive care with rehydration and electrolyte replacement.
The minor heat illnesses are benign and self-limited with no on-going complications. For the more moderate heat illnesses, including heat exhaustion/stress and heat injury, complications are exacerbated by the patient's age, comorbidities, the presence of organ injury, or if severe electrolyte disturbances have occurred. These patients may benefit from admission to a tertiary care center for further stabilization before discharge. Predictive factors for hospitalization include age of equal to or greater than 65 years, the level of temperature elevation, alterations in mental status, and elevations of creatinine from baseline.
Public education should focus on preventative measures that are possible to ensure people understand the risks of heat illness. Education should include the importance of maintaining an adequate level of cooling during the hottest of days and the importance of staying shaded. Utilizing the wet-bulb globe temperature (WBGT) index, which is a composite index of temperature, humidity, and solar radiation is an excellent assessment of the environmental risk of developing a heat-related illness. If not available, heat index, which accounts for temperature and humidity, can be used as a slightly less effective surrogate. Screening tools also exist both at the personal and at the population-level to evaluate for susceptibility to heat illness, although there is conflicting evidence about their effectiveness. Other preventative strategies include utilizing air-conditioning, taking breaks in shaded or cooled areas during particularly hot days, and avoidance of excessive exertion during the hottest periods of the day. Acclimation in the days to weeks before performing maximal exertion at a hot location or time of day is one of the most successful strategies to prevent heat-related injuries. Previous recommendations include an acclimation period of 7 to 14 days with maintenance as decay can occur over 2 to 3 weeks. Appropriate hydration is crucial in hot environments, with one recommendation in adolescents being to consume 240 mL (8 ounces) of fluid for every pound lost during exercise.
An interprofessional approach to patient care is unavoidable as there must be coordination between bystanders or staff at the scene of the injury and first responders when they arrive. Treatment for these conditions can occur at the scene and may include removal from a hot environment, supine positioning, and rehydration. Once evaluated by an initial medical professional like an on-site physical trainer or by emergency medical services, transport concerns may require discussion with an online medical control physician depending on the stability of the patient. Also, specific centers may be more appropriate for the care of the patient depending on the availability of additional resources, including critical care units. If transport is necessary for a patient, different treatment modalities may need to be employed, including EKG testing, glucose point-of-care testing, and IV placement by technicians or nursing staff, or more invasive cooling measures depending on the severity of the patient's illness that may require specialist consultation. Emergency department nurses are involved in triage, initial care, patient monitoring, and education. Depending on the degree of dysfunction, specialists in critical care may also need to be consulted, although this would be more consistent with heatstroke. With an interprofessional approach to these cases, heat-related illness can achieve optimal patient outcomes. [Level 5]
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